A free-piston-type Stirling-cycle engine designed for the production of cold is also called a reversed-Stirling-cycle engine in terms of the thermodynamic cycle on which it is based. FIG. 10 shows a sectional view of a conventional Stirling-cycle engine. A typical Stirling-cycle engine has a piston 1 and a displacer 2 that reciprocate linearly inside a cylinder 3. The piston 1 and the displacer 2 are arranged coaxially, and a rod 2a formed on the displacer 2 penetrates the piston 1 through a slide hole 1a formed at its center. Thus, the piston 1 and the displacer 2 are smoothly slidable along the cylinder inner slide surface 3a. Moreover, the piston 1 and the displacer 2 are resiliently supported on a pressure vessel 4 by a piston support spring 5 and a displacer support spring 6 respectively.
The space formed by the cylinder 3 is divided into two spaces by the piston 1. One of these spaces is a working space (a first and a third spaces) 7 located on that side of the piston 1 which faces the displacer 2, and the other is a back space (a second space) 8 located on that side of the piston 1 which faces away from the displacer 2. These spaces are filled with a working medium such as helium gas. The piston 1 is made to reciprocate with a predetermined period by an piston driving member, not shown, such as a linear motor. Thus, the working medium is compressed or expanded in the working space 7. The displacer 2 is made to reciprocate linearly by the change in the pressure of the working medium compressed or expanded in the working space 7. Here, the piston 1 and the displacer 2 are typically so designed as to reciprocate with the same period but with a 90°.
The working space 7 is further divided into two spaces by the displacer 2. One of these spaces is the first space 7a located between the piston 1 and the displacer 2, and the other is the third space 7b located at the tip of the cylinder 3. These two spaces are connected together through a regenerator 9, which is typically formed out of copper in the form of mesh. The working medium in the third space 7b produces cold at the tip, called the cold head, of the cylinder 3. The principles of the reversed Stirling thermodynamic cycle, as by what mechanism cold is produced here, are well known, and therefore no explanation will be given in these respects.
Between the cylinder slide surface 3a and the piston slide surface 1b, a sealing means, not shown, for shielding the first space 7a and the second space 8 from each other is provided. As the shielding means is typically used an inexpensive seal ring with a simple structure. However, owing to various factors, such as the influence of thermal expansion and the wear of the seal ring after a prolonged period of operation, perfect sealing is impossible here, and thus a very small gap appears between the cylinder slide surface 3a and the piston slide surface 1b. When the engine is driven, the piston 1 reciprocates and produces changes in the pressure of the working medium in the first space 7a and the second space 8. The resulting pressure difference between these spaces causes the working medium to flow from one of those spaces to the other through the aforementioned very small gap. Specifically, when the pressure in the first space 7a is higher than that in the second space 8, the working medium flows from the first space 7a to the second space 8; when the pressure in the second space 8 is higher than that in the first space 7a, the working medium flows from the second space 8 to the first space 7a. 
The very small gap appearing between the cylinder slide surface 3a and the piston slide surface 1b is not constant but varies according to the surface conditions of the slide components and the contact, wear, and other conditions of the seal ring. Thus, the inflow volume and the outflow volume of the working medium to and from the second space 8 as seen from the first space 7a are not exactly equal. For this reason, when the engine is driven continuously and the working medium leaks little by little from the first space 7a to the second space 8, the center position of the piston 1, which has initially been set so that equilibrium of pressure is achieved between the first space 7a and the second space 8, gradually moves toward the first space 7a, where the pressure is lower now. As a result, as the pressure of the working medium in the first space 7a lowers, problems arise, such as lower cooling performance than expected, or collision between the piston 1 and the displacer 2 resulting from the center position of the reciprocating movement of the piston 1 being displaced from the initially set position.
These problems may be solved by increasing the spring constant of the piston support spring 5 so as to increase the force supporting the piston 1. This, however, is not at all effective against the leakage of the working medium out of the first space 7a, and only invites an increase in the power required by the driving means for driving the piston 1, leading to an increase in the input electric power. This leads to another problem, lower cooling efficiency.
To solve this problem, Japanese Patent Application Laid-Open No. 2000-39222 discloses a method of reducing the variation of the center position of the reciprocating movement of the piston 1 by maintaining the equilibrium of pressure between the first space 7a and the second space 8. FIG. 11A shows a sectional view of the Stirling-cycle engine disclosed in Japanese Patent Application Laid-Open No. 2000-39222. The structure here is the same as that shown in FIG. 10 except for the shape of part of the piston 1. FIG. 11B is a perspective view of a portion around the piston 1 when the piston 1 is in the center position of its reciprocating movement as initially set. In the piston 1 are formed a first groove 10a that connects to the first space and runs in the direction of the reciprocating movement X of the piston 1 and a second groove 10b that is continuous with the first groove 10a and runs at an angle (in the figure, 90°) to the direction of the reciprocating movement X of the piston 1. In the cylinder 3 is formed a circular hole 12 that penetrates from the second groove 10b to the second space 8. The moment that the second groove 10b coincides with the circular hole 12 during the operation of the piston 1, the first space 7a and the second space 8 are momentarily connected together and permit the working medium to flow. This brings the pressure in those two spaces into equilibrium and thereby permits the piston 1 to reciprocate with the center position kept in the initially set position.
As described above, connecting the first space 7a and the second space 8 together by way of a minute flow passage when the piston 1 is in the center position of its reciprocating movement as initially set is effective in keeping the center position in the initially set position. However, to achieve higher cooling performance, it is necessary either to increase the number of cycles of the reciprocating movement of the piston 1 or to increase the amplitude of each cycle of the reciprocating movement of the piston 1. This inevitably increases the flow rate of the working medium between the first space 7a and the second space 8, and thus requires that the first and second grooves 10a and 10b have a larger cross-sectional area. Here, simply increasing the dimensions and cross-sectional area of the first and second grooves 10a and 10b makes the area over which the second groove 10b communicates with the second space 8 larger with respect to the working stroke of the piston 1, and also makes the time for which they communicate with each other longer.
Thus, it is possible to achieve equilibrium of pressure between the first space 7a and the second space 8, but it is not possible to keep the center position of the reciprocating movement of the piston 1 accurately in the initially set position. Moreover, a gas flow loss occurs in the first and second grooves 10a and 10b. This increases the input of power required to operate the piston 1, and thus makes it impossible to enhance the performance of the Stirling-cycle engine as expected.